This invention relates photoluminescent materials (or phosphors) and lighting technology. In particular, this invention relates to phosphors containing borate of rare-earth metals, and to light sources incorporating such phosphors.
A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range. Well-known phosphors have been used in mercury vapor discharge lamps to convert ultraviolet (“UV”) radiation emitted by the excited mercury vapor to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons (used in cathode ray tubes) or X rays (for example, scintillators in X-ray detection systems).
Fluorescent lamps having high luminous output and color rendering indices (“CRI”), which are based on mercury discharge and used for illumination, typically include three phosphors that convert UV radiation of the mercury discharge into relatively narrow bands of blue, green, and red visible light, concentrated in the spectral regions where the human eye has the highest sensitivity (450, 540, and 610 nm). Lanthanum phosphate co-activated with terbium and cerium (LaPO4:Tb3+,Ce3+) has been a favorite green light-emitting phosphor, having a large peak emission at about 543 nm. Ce3+ ions act as an efficient sensitizer to Tb3+ ions. Energy is transferred from Ce3+ ions to Tb3+ ions, thus giving an efficient emission under UV excitation. However, the UV output is still relatively high because the Ce3+—Ce3+ energy transfer is more than the Ce3+−Tb3+ energy transfer. Such UV output tends to reduce the efficiency of this phosphor.
Therefore, there is till a need to provide more efficient green light-emitting phosphors. It is also very desirable to use such green light-emitting phosphors to produce light sources having high energy efficiency and high CRIs.